Evidence-Based Evaluation of Popular Weight Loss
Diets

Text-only Preview

According to the Centers for Disease Control, one out of two adult Americans exceeds the upper healthy weight limit (body mass index or BMI > 25). Moreover, 20% of US adults are clinically obese (BMI > 30). Health problems associated with our country's obesity epidemic are well established: type 2 diabetes, cardiovascular disease, stroke, gallbladder disease, osteoarthritis, sleep apnea and possibly cancer. The 61% increase in the incidence of US adult obesity from 1991 to 2000 has coincided with a 49% rise in type 2 diabetes among adult Americans.

The proliferation of top-selling diet books published in the past five years is an ironic twist to the health concerns among overweight Americans striving to lose weight. These popular diets typically offer dieters "empty" promises such as quick weight loss, increased energy level and absence of hunger/food deprivation. But few dieters actually try to improve their eating habits. The Centers for Disease Control report that only 17.5% of dieters try to consume fewer calories and increase physical activity, two key factors for weight control. Furthermore, approximately one out of four dieters consumes the recommended five servings of fruits and vegetables daily. Considering that dieters fail to practice sound weight loss practices, approximately 70% of these individuals regain at least half of their weight loss within two years.

Despite the apparent poor success rate of fad diets, these reducing eating plans continue to grow in popularity among overweight Americans wishing to shed some pounds. In fact, over half of the 50 best-selling diet books have been published since 1999.

What is the "magic" bullet for successful weight loss? Researchers from the United States Department of Agriculture (USDA) and other institutions have conducted an extensive review of scientific literature to evaluate the efficacy of popular diets. Based on their analysis of existing data collected on reducing diets, calorie restriction in itself is the "key" ingredient for successful weight loss. Unfortunately, evidence for the long-term health safety and maintenance of weight loss associated with fad diets remains unclear. Highlights of research findings on popular diets are described below.

1. CALORIES:Eating less calories leads to weight loss.

• Total daily caloric intakeaveraging 1400-1500 calories daily results in weight lossregardless of the macronutrient composition (i.e. breakdown of protein, carbohydrate and fat) of the weight-reducing diet. Although physical activity can enhance weight loss, dieters who adhered to a 1500 calorie/day diet still lost weight because they expended more energy than that consumed.

• Dieters who adhered to a low-fat, high carbohydrate diet rich in fruits, vegetables and grains consumed less calories than those on other types of diets (see table below). In short, weight loss study results indicate an inverse relationship between carbohydrate intake and body weight; heavier people consistently consume less carbohydrate. These findings dispel the myth among dieters that carbohydrates are "fattening".

• High-fat, low-carbohydrate diets are deficient in several nutrients. These regimens are consistently low in vitamins A, B6, D and E, thiamin, folate, calcium, magnesium, iron, potassium and dietary fiber. Hence, supplementation is highly recommended for those who adhere to this type of weight-reducing regimen.

• Moderate-fat, low-carbohydrate diets are nutritionally balanced per the USDA Food Guide Pyramid guidelines. If one or more of the six food groups (i.e. grains, fruits, vegetables, meats, dairy and fats & sugars) is eliminated or severely restricted, inadequate intake of nutrients provided from that food category is likely to occur.

• Low-fat, high-carbohydrate diets are often deficient in vitamin E, vitamin B12 and zinc. Eating foods fortified with these nutrients or a supplementation may be necessary for those adhering to low-and very-low fat regimens.

• Weight loss in itself results in loss in body weight, body fat and even lean muscle mass. Total daily calorie restriction (~1500 calories/day) sustained over a period of time promotes a loss of body fat, despite the macronutrient composition of the diet.

4. Physiological Changes During Weight Loss • Weight loss is directly correlated with a decrease in (1) total blood cholesterol, (2) LDL-cholesterol and (3) plasma triglycerides. Greater reduction in LDL-cholesterol is observed during weight loss with a low saturated fat intake. The types of dietary fat consumed and duration of the weight loss program affected HDL-cholesterol levels. • Caloric restriction regardless of the macronutrient content of the weight loss diet improves glycemic control by lowering blood sugar and insulin levels.

• Weight loss in itself decreases blood pressure. However, in the absence of weight loss, the DASH diet (a low-fat diet rich in fruits, vegetables, whole grains and low-fat dairy foods) promotes a reduction in blood pressure. This effect is magnified when the DASH diet is combined with a low sodium intake.

5. Hunger, Satiety and Adherence to Weight Loss Diets

• Insulin and leptin regulate long-term weight regulation. Insulin stimulates the uptake of glucose and proteins into the cells as well as possibly increasing appetite. Leptin, on the other hand, is a hormone released from fat cells, which helps suppress appetite and increase metabolism. Insulin and leptin secretion is influenced by the macronutrient composition of the diet although the actual mechanisms remain unclear. During weight loss, blood insulin and leptin levels drop accordingly.

• All fat-restricted diets offer satiety value, contrary to claims made by high-protein and high fat diet gurus. In fact, dieters who follow both a calorie and fat-restricted regimen commonly complain of having “too much food to eat”. This may be due to the high fiber, high water content of low-fat foods, which may account for dieters’ enhanced feeling of fullness during and between meals. Additionally, those who adhere to a low-fat regimen over time appear to develop an aversion to fatty foods, which may also contribute to long-term success in weight control. Furthermore, high dietary fiber intake is correlated with lower insulin levels and long-term maintenance of weight loss.